In the past, ship hull inspections have been required in order to obtain certification from governmental authorities as to the seaworthiness of the vessel.
The most common way to obtain the governmental certificate is to dry-dock the vessel, an expensive process, and to both visually and ultrasonically inspect the hull of the vessel when it is in dry dock. The procedure takes the vessel out of service for as much as two weeks and requires as many as five to ten technicians in order to survey the hull.
Typically in such a case, a surveying instrument is run over the surface of the hull, with the position of the surveying instrument being mechanically measured from a fiducial point on the hull. The accuracy of the measured point on the hull is a function of how accurate the manual surveying is, which is a time-consuming process. One generally has to measure the distance of the head of the surveying instrument to the fiducial point utilizing mechanical gauges, measuring rods or bars and the like, with position measurements being subject to error.
As a result of the difficulty of accurately specifying the location of the surveying instrument, surveyors typically measure out a one-foot grid across the surface of the hull and take measurements at the cross-points. The problem, however, is that with one-foot spacing, wasting and other defects in the hull are oftentimes missed if they occur between the cross-points.
In an effort to avoid dry-docking the vessel, divers are sometimes sent down to survey the hull by swimming about the hull and measuring hull thickness and wasting at various points. The location of the diver making the survey and thus the points surveyed is likewise problematic, with physical measurements of a surveyed point underwater being much more difficult than when the ship is hauled out. To help in specifying the measurement points, in the past sonar techniques were used to locate the diver. However, positional inaccuracies are in the one-meter range.
Thus, whether there is an in-water survey through the utilization of divers or whether the vessel is hauled out for the survey, in the past, surveys have suffered from the fact that the number of actual measurements made on the hull is limited, meaning that there are areas of the hull which remain unsurveyed. Wasting, sometimes underneath support structures within the hull and at places where they are not visible, contribute to the unseaworthiness of the vessel. Historically, vessels within weeks of having been surveyed have broken up at sea due to the fact that wasting and other defects were not uncovered by the survey.
In order to obtain better in-water measurements, ships have been tied up to piers or are placed at anchor. However, it is impossible to render the ship motionless against winds, tides and current, even if anchored bow and stern. There is always a certain amount of motion of the vessel due to currents and wave action so that depending on the size of the vessel, the hull may move by many meters, which directly impacts trying to ascertain where the survey measurements were made.
In general, the measurements are made at inspection points on the hull which are either visual or sonic inspections of hull plating, hull thickness, weld integrity, cathodic erosion and other hull weaknesses for which one seeks to have a map of the defects so that one can repair the hull at the point where the defect is detected.
Were it possible to accurately determine the position of the hull vis-à-vis the sensor used to detect the hull defect and were it possible to make measurements in a virtually continuous fashion, then the hull could be completely and accurately inspected. This in turn requires a system that is motion-compensated.